The present invention relates to lasers and, more particularly, to a gas laser plasma tube having a gas return path within the structure defining the discharge cavity, which return path is of a design inhibiting gaseous discharge along such path even in relatively high powered lasers.
Ion gas lasers produce coherent radiation from an electrical gaseous discharge. Such gaseous discharge takes place within a plasma tube having structure defining a discharge cavity between anode and cathode assemblies. Such structure typically is a cylinder of a high temperature material, such as ceramic, positioned between the anode and cathode, and the discharge cavity is an axial bore extending through such cylinder.
The gaseous discharge causes gas to flow through the discharge cavity from one end to the other. It is therefore common practice to include structure of one sort or another defining a gas return path extending between the ends of the discharge cavity to allow the pressures at such ends to be equalized. Traditionally, the gas return path in most lasers has been defined by structure, such as glass tubing, provided as an external appendage to the plasma tube. Such external arrangements, though, are relatively fragile and are not uncommonly broken. To avoid such fragility, lasers are now made incorporating the gas return path directly into the structure of the plasma tube which defines the discharge cavity. In those lasers in which the structure providing the discharge cavity is a cylinder of a high temperature material, the gas return path typically is provided by one or more bores which extend through the cylinder parallel to the discharge cavity. Care must be taken with such a structure, however, to assure that the gaseous discharge is confined to the discharge cavity and is not also initiated along the gas return path bores. Various approaches have been used in the past to so restrict the gaseous discharge. U.S. Pat. No. 3,624,543 describes one approach in which the gas return path bores generally have a smaller transverse cross-sectional area than the discharge cavity. Since the gaseous discharge will tend to follow the path of least resistance between the anode and cathode assemblies, such cross-sectional relationship will cause the discharge to take place preferentially in the discharge cavity. This approach is not entirely suitable, however, for relatively high powered lasers, i.e., lasers having an output power rating of 2 watts or more. That is, the same reduction in cross-section of the gas return paths used to inhibit gaseous discharge also inhibits gas return flow therethrough. Besides higher powered lasers having a greater anode-cathode potential differential making discharge in an internal gas return path more likely, they typically have a higher rate of gas flow through the discharge cavity and it is difficult to obtain sufficient return gas flow for smooth operation. Another approach to preventing discharge in internal gas return path bores has been to insert restrictions, such as mesh or thin discs, in the return paths to inhibit gaseous discharge. Again, such restrictions also may tend to inhibit gas flow and reduce the effectiveness of the path to return gas as intended.